tion by the interposition of this plate of quartz between the reflecting surface and the Nicol. The changes in the light are no longer mere alterations of brightness, but exhibit a succession of colors resembling in their main features those of the rainbow or spectrum.
The peculiar condition to which light must be brought in order that these phenomena may be produced is called polarization; and, although an explanation of its nature must be reserved until later, I beg you to notice that it is effected in this instance by reflection from a plate of glass. A similar effect is produced if light be reflected from many other substances, such as the leaves of trees, particularly ivy, mahogany furniture, windows, shutters, and often roofs of houses, oil-paintings, etc., and last, but not least, the surface of water. In each of these cases the alternations of light and darkness are most strongly marked, and the colors (if a quartz plate be used) are most vivid, or, in technical language, the polarization is most complete, when the light is reflected from each substance at a particular angle. In proportion as the inclination of the light deviates from this angle the colors become fainter, until, when it deviates very greatly, all trace of polarization at last disappears. Without occupying the time necessary to shift our apparatus so as to exhibit this with the glass plate, we may alter the reflecting surface from glass to water, and, by projecting on the screen the beautiful phenomena of liquid waves, make visible the different degrees of polarization produced at the variously-inclined portions of the surfaces of those waves. A tea-tray will serve as well as any thing else to form our little sea, and a periodic tap at one corner will cause ripple enough for our present purpose. The waves now appear bright on the screen, and, although brighter in some parts than in others, they are nowhere entirely dark. But on turning round the Nicol the contrast of light and darkness becomes much stronger than before. Here and there the light is absolutely extinguished; in these parts the polarization is complete, in others incomplete in various degrees. And if the quartz plate be again introduced we have the beautiful phenomena of iris-colored rings playing over the surface of our miniature sea.
Now, that which you see here produced by our lamp and tea-tray, you may see any day under the bright sky of this southern coast. By using an apparatus such as we have here, or a simpler one which I will immediately describe, you may bring out for yourselves these phenomena of color, and thereby detect the profusion of polarization which Nature sheds around us. But, before describing it, there is one peculiar feature of all these experiments which must be noticed namely, that the same results would be produced if we changed the positions of the lamp and the screen. The light which is now polarized by the glass or the water, and examined by the Nicol, might equally well be polarized by the Nicol and examined by the glass or the water. And, therefore, if we find that any contrivance will serve